Generally speaking, a Laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The working mechanism of Laser is that a standing wave and a luminous body co-operates to generate resonances in a resonant cavity, wherein light beams illuminate forward and backward within the cavity to stimulate electrons with high energy of the luminous body, so that a light beam with high energy density is formed and radiates out of the resonant cavity in a straight line. The light beam with high energy density, what is called Laser light beam, differs from other light sources because of Laser emits light coherently. Spatial coherence allows Laser to be focused to a tight spot, and also allows a Laser light beam to stay narrow over great or long distances, what is called collimation effect. Lasers can also have high temporal coherence, which allows them to emit light with a very narrow spectrum, i.e., they can emit a single color of light. Temporal coherence can be used to produce pulses of light as short as a femtosecond. As the above mentioned, Lasers have many special characters of high brightness, good monochromaticity, and good optical coherence, etc. In addition, Lasers can keep good illumination quality. Therefore, Lasers are currently widely applied to a variety of industries because of the above-mentioned characters. For example, Lasers are applied used in a laser displacement interferometer, which is an optical system using wavelength as the unit of measurement or counting, and is a kind of displacement measurement instrument. Displacement measurement instruments are commonly used in industries, such as precision measurement industry or precision machinery industry, for the calibration of positional accuracy, such as numerically controlled (NC) machine tools or linear motion positioning stages.
A laser consists of a gain medium, a mechanism to energize it, and something to provide optical feedback. The gain medium is a material with properties that allow it to amplify light by way of stimulated emission. Light of a specific wavelength that passes through the gain medium is amplified (increases in power). For the gain medium to amplify light, it needs to be supplied with energy in a process called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Pump light may be provided by a flash lamp or by another laser. As the above mentioned, Lasers are very sensitive to environmental temperatures. When environmental temperature changes, the length of the resonant cavity of Laser varies accordingly. For example, the length of the resonant cavity of Laser is expanded or extended when the environmental temperatures are getting hotter or getting higher, or, on the contrary, the length of the resonant cavity of Laser is shrunk or shortened when the environmental temperatures are getting colder or getting lower. The wavelength of Laser is accordingly drifted or changed owing to the above-mentioned phenomenon that the length of the resonant cavity of Laser changes. Therefore, a variety of frequency stabilization techniques are developed for Lasers, which can reduce and stabilize the variations of wavelengths of Lasers and improve the stabilization of wavelengths of Lasers for resolving the problems mentioned above. However, prior frequency stabilization techniques have many shortcomings, such as complicated system, high cost, and low efficiency, etc. The problems of wavelength change or drift of Lasers are still serious and not resolved. It is known that the wavelength of laser in air is equal to its wavelength in vacuum divided by the refractive index of air. At any ambient condition, the actual wavelength of laser has to be corrected by the refractive index of air, which is a function of temperature, relative humidity and pressure. Common laser interferometers all require an air sensor that detect the current temperature, relative humidity and atmospheric pressure and correct the refractive index of air through a complicate formula, such as the Edlen equation, Ciddor equation or modified Edlen equation by Birch. These equations are all empirical equations for He—Ne Lasers and may not be suitable for Laser diode. Therefore, further techniques are necessarily demanded for adjusting and stabilizing the wavelength change or drift of Lasers efficiently.
Please referring to FIGS. 1 and 2, FIG. 1 shows a prior optical system of a real-time wavelength correction system for visible light, and FIG. 2 shows prior parallel light beam of first-order diffraction focusing on a light position sensing detector (PSD). As shown in FIGS. 1 and 2, a real-time wavelength correction system for visible light PA100 is collocated and operated with an optical system PA200. The optical system PA200 comprises a parallel light emitter PA201, a displacement interferometer module PA202 and a movable reflector PA203.
The parallel light emitter PA201 emits a parallel light beam (not marked in FIGS. 1 and 2) in a first direction L1. The displacement interferometer module PA202 is set on one side of the parallel light emitter PA201 along the first direction L1, so that the parallel light beam can pass through the displacement interferometer module PA202. The movable reflector PA203 is set on one side of the displacement interferometer module PA202 along the first direction L1 corresponding to the parallel light emitter PA201, so that the movable reflector PA203 can receive and reflect the parallel light beam passed from the displacement interferometer module PA202 along the first direction L1. Furthermore, the parallel light beam has a reference wavelength under a standard environmental state. For example, the standard environmental state is that the atmospheric pressure is at one atmospheric pressure (ATM) and the temperature is at 20 centigrade degrees. In one preferred embodiment, the optical system PA200 is preferred a laser displacement interferometer, the parallel light emitter PA201 is preferred a laser beam emitter, and the parallel light beam is accordingly preferred a Laser beam.
Generally speaking, the prior real-time wavelength correction system for visible light PA100 includes an optical beam splitter PA1, a diffractive grating PA2, a focusing lens PA3, a light spot position sensing detector PA4, and a supporting platform PA5.
The optical beam splitter PA1 is set between the parallel light emitter PA201 and the displacement interferometer module PA202 along the first direction L1 and the light path of parallel light beam. The parallel light beam can pass through the optical beam splitter PA1 and then pass through the displacement interferometer module PA202 along the first direction L1. The parallel light beam can be split into two beams, including a new split parallel light beam and the original parallel light beam, during passing through the optical beam splitter PA1 by split effect of the optical beam splitter PA1. The second direction L2 of the split parallel light beam is perpendicular to the first direction L1 of the original parallel light beam.
The diffractive grating PA2 is set on one side of the optical beam splitter PA1, for receiving the split parallel light beam split from the original parallel light beam, wherein the split parallel light beam is perpendicular to the original parallel light beam. Then, the split parallel light beam is diffracted by the diffractive grating PA2, and accordingly forms or generates a first-order diffractive parallel light beam PA2013 (shown in FIG. 2) in a diffraction direction L21 (shown in FIG. 1) by diffracting effect of the diffractive grating PA2. In one embodiment, the diffractive grating PA2 is preferred a transmission diffractive grating. The split parallel light beam can be diffracted into a variety of light beam in a variety of different radiation directions during passing through the diffractive grating PA2 from the second direction L2 under the standard environment state such as the atmospheric pressure at one atmospheric pressure (ATM) and the temperature at 20 centigrade degrees. In one embodiment, the first-order diffractive parallel light beam PA2013 (shown in FIG. 2) is preferred positive first-order diffractive parallel light beam.
According to the above mentioned, the diffraction angle of the positive first-order diffractive parallel light beam corresponding to the diffractive grating PA2 can be calculated according to the following prior diffraction equation:d(sin θl+sin θq)=λ  (1)Wherein d is the grating space, λ is the wavelength of the parallel light beam, θl is the incidence angle of the split parallel light beam in the second direction L2 and θq is the diffraction angle of the first-order diffractive parallel light beam PA2013 in a diffraction direction L21. When the incidence angle θl of the split parallel light beam is adjusted to zero, the positive first-order diffraction angle can be calculated according to the following equation:
                              θ          q                =                              sin                          -              1                                ⁡                      (                          λ              d                        )                                              (        2        )            
It is supposed that the wavelength of the reference first-order diffractive parallel light beam PA2013s and the split parallel light beam is a reference wavelength under a standard environment state (that is, the atmospheric pressure is at one atmospheric pressure (ATM) and the temperature is at 20 centigrade degrees), and the wavelength of the first-order diffractive parallel light beam PA2013 is an actual wavelength under a general environment state. Therefore, a difference of angle value Δθ1 between the reference first-order diffractive parallel light beam PA2013s and the first-order diffractive parallel light beam PA2013 can be easily calculated by comparing the reference wavelength with the actual wavelength. Furthermore, the diffraction angle value can be calculated by the operation computer of the optical system PA200 according to the autocollimator principle and the following equation:Δx=f tan(Δθ1)  (3)Wherein, Δx is the difference between a light spot position parameter and a reference light spot position parameter, and f is the focal length of the focusing lens PA3.
Referring to FIG. 2, the focusing lens PA3 is used for receiving the first-order diffractive parallel light beam PA2013 diffracted from the diffractive grating PA2 and focusing the first-order diffractive parallel light beam PA2013 on a back focal plane of the focusing lens PA3 (not marked in FIG. 2). Consequently, the first-order diffractive parallel light beam PA2013 can focus on the back focal plane of the focusing lens PA3 to form a light spot AP1, wherein, the back focal plane is a plane formed by assembling all of the focus light spots which pass from the first-order diffractive parallel light beam PA2013 through the focusing lens PA3 and focus on the back focal plane of the focusing lens PA3.
The light spot position sensing detector PA4 is electrically connected to the operation computer of the optical system PA200, and the he light spot position sensing detector PA4 is set on the same place of the back focal plane of the focusing lens PA3. The light spot position sensing detector PA4 includes a sensing plane PA41, and the sensing plane PA41 overlaps the back focal plane of the focusing lens PA3. The sensing plane PA41 of the light spot position sensing detector PA4 is used for sending the forming place of the light spot AP1, so that the light spot position sensing detector PA4 can generate a light spot position parameter according to the forming place of the light spot AP1.
The focusing lens PA3 and the light spot position sensing detector PA4 are set on a supporting platform PA5. The supporting platform PA5 is provided for fixedly setting the focusing lens PA3 and the light spot position sensing detector PA4. Therefore, the light spot position sensing detector PA4 can be fixedly set and arranged on the supporting platform PA5 according to the focal length of the focusing lens PA3. In addition, the supporting platform PA5 is provided for adjusting the positions of the focusing lens PA3, the light spot position sensing detector PA4, and the diffractive grating PA2 simultaneously, and adjusting the distances between the focusing lens PA3, the light spot position sensing detector PA4, and the diffractive grating PA2 simultaneously.
As the above mentioned, the first-order diffractive parallel light beam PA2013 (shown in FIG. 2) is formed by the parallel light beam passing through the diffractive grating PA2 and then diffracted by the diffractive grating PA2, wherein a diffraction angle is generated between the first-order diffractive parallel light beam PA2013 and the normal line of the diffractive grating PA2 under a general environmental state. The first-order diffractive parallel light beam PA2013 passes through the focusing lens PA3, and then, the first-order diffractive parallel light beam PA2013 focuses on a back focal plane of the focusing lens PA3 (not marked in FIG. 2). Consequently, the first-order diffractive parallel light beam PA2013 can be focused by the focusing lens PA3 on the back focal plane of the focusing lens PA3 to form a light spot AP1. The light spot AP1 is used for calculating a light spot position parameter. On the other hand, a reference first-order diffractive parallel light beam PA2013s (shown in FIG. 2) is formed by the parallel light beam passing through the diffractive grating PA2 and then diffracted by the diffractive grating PA2 under a standard environment state. The reference first-order diffractive parallel light beam PA2013s passes through the focusing lens PA3, and then, the first-order diffractive parallel light beam PA2013 focuses on a back focal plane of the focusing lens PA3 (not marked in FIG. 2). That is, the reference first-order diffractive parallel light beam PA2013s can be focused by the focusing lens PA3 on the back focal plane of the focusing lens PA3 to form a light spot AP2, wherein so that the light spot position sensing detector PA4 can generate a reference light spot position parameter according to the forming place of the light spot AP2. The difference between the light spot position parameter and the reference light spot position parameter can be used to calculate a difference of angle value Δθ1 between the reference first-order diffractive parallel light beam PA2013s and the first-order diffractive parallel light beam PA2013 by comparing the reference wavelength with the actual wavelength. Accordingly, the reference wavelength can be corrected and a real-time correction actual wavelength is then generated according to the difference of angle value Δθ1.
As the above mentioned, the prior real-time wavelength correction system for visible light PA100 can correct the change of the wavelength of the parallel light beam due to the change of environment conditions by using optical diffraction principle. However, the temperatures of the parallel light emitter PA201 may rise because of the parallel light emitter PA201 emits the parallel light beam continuously for a long time. The changes of temperatures of the parallel light emitter PA201 will also result in the drift or change of the wavelength of the parallel light beam. Therefore, the performance of the prior real-time wavelength correction system will be weakened and downgraded. The incidence angle of the parallel light beam emitting from the parallel light emitter PA201 to the optical beam splitter PA1 will be changed or drifted, so that the incidence angle of the split parallel light beam split from the optical beam splitter PA1 to the diffractive grating PA2 will also be changed or drifted. The changed or drifted incidence angle of the split parallel light beam split from the optical beam splitter PA1 to the diffractive grating PA2 will result in the change or drift of the difference of angle value Δθ1. Therefore, the accuracy of a real-time wavelength correction value by using the prior real-time wavelength correction system, which performs a real-time wavelength correction by using the difference of angle value Δθ1, will be reduced.